1. Field of the Invention
The present invention relates to an electrophotographic photoconductor and a color image forming device. In particular, it relates to an electrophotographic photoconductor which shows a small variation in sensitivity and exhibits a high sensitivity even at a small light exposure and a tandem-system color image forming device provided therewith.
2. Description of the Related Art
Heretofore, color image forming devices using an endless belt-shaped rotating member entrained on a plurality of rollers have been proposed (see, for example, patent document 1).
In such an image forming device, an intermediate transfer body for primarily transferring a toner image formed on an image carrier by an electrophotographic system and then secondarily transferring the image to a transfer material is constituted from a belt-shaped rotating member (intermediate transfer belt). A tandem system is adopted that has a color printing function to form color images by superimposing toners of a plurality of colors such as yellow (Y), magenta (M), cyan (C) and black (K), on an intermediate transfer belt. Therefore, in such a color image forming device, developing devices each corresponding to an individual color are arranged along the intermediate transfer belt in order to superimpose toners of a plurality of colors.
On the intermediate transfer belt, toner images of four colors, namely YMCK, are transferred (primarily transferred) one after another so that they are superimposed one on another by each photoconductor drum of the developing device and thereby a color image is formed. Furthermore, the color image formed on the intermediate transfer belt is transferred (secondarily transformed) onto a transfer material such as a paper sheet by a secondary transfer roller arranged facing the intermediate transfer belt, thereby forming a predetermined color image. [Patent document 1] JP-2005-43593A (Claims)
In the color image forming device disclosed in patent documents 1, however, variation in sensitivity easily occurs among four electrophotographic photoconductors corresponding to the above-mentioned four-color toner development and there is a problem that it is difficult to form favorable images.
In particular, when the rotation speed of an electrophotographic photoconductor is a predetermined value or more, for example 70 rpm or more, or when the electrophotographic photoconductor to be mounted has an outer diameter of 30 mm or less, the exposure/development times become shortened and the amount of light exposure decreases. For this reason, there is a problem that the variation in sensitivity among the four electrophotographic photoconductors occurs extremely easily.
Under such circumstances, attempts have been made to improve print properties of color images by increasing the light exposure strength. However, there are further problems that the light degradation of electrophotographic photoconductors are promoted, resulting in great deterioration of durability or the cost or scale of exposure devices increases.
Therefore, an appropriate parameter for making the variations in sensitivity uniform among four electrophotographic photoconductors has been demanded, but only the sensitivity has been standardized according to the value of light potential or the like.